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THE CRYSTAL STRUCTURE of FENCOOPERITE: UNIQUE [Fe3+

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THE CRYSTAL STRUCTURE of FENCOOPERITE: UNIQUE [Fe3+ 1065 The Canadian Mineralogist Vol. 39, pp. 1065-1071 (2001) THE CRYSTAL STRUCTURE OF FENCOOPERITE: 3+ UNIQUE [Fe 3O13] PINWHEELS CROSS-CONNECTED BY [Si8O22] ISLANDS JOEL D. GRICE§ Research Division, Canadian Museum of Nature, P.O. Box 3443, Station D, Ottawa, Ontario K1P 6P4, Canada ABSTRACT 3+ The crystal structure of fencooperite, ideally Ba6Fe 3Si8O23(CO3)2Cl3•H2O, has been solved in the trigonal space-group P3m1, with a 10.7409(5), c 7.0955(4) Å, V 708.9(1) Å3 and Z = 1. The final residual index for observed reflections is 0.038. The uniqueness of the fencooperite structure is a result of two previously unknown fundamental building blocks (FBB). The islands of silica tetrahedra form [Si8O22] units described as double open-branched triple tetrahedra sharing vertices along [001]. The 3+ other unique FBB consists of three (Fe O5) tetragonal pyramids having one of the O atoms common to all three polyhedra, forming a pinwheel trimer [Fe3O13]. The pronounced optical absorption is attributed to this coordination complex of iron. This trimer cross-links the [Si8O22] islands to form a framework structure. The framework supports intersecting tunnels in all three crystallographic directions that are filled by Ba and Cl atoms and by H2O and CO3 groups. The structure of fencooperite is compared to that of gillespite, pellyite, titantaramellite and orthoericssonite. In gillespite and pellyite, individual polyhedra of Fe2+ in four-fold coordination with oxygen cross-link sheets and chains of silica tetrahedra, whereas the chains of Fe3+ in (titan)taramellite having six-fold coordination with oxygen cross-links island rings of silica tetrahedra. In orthoericssonite, which 3+ 3+ has the same five-fold coordination of Fe as fencooperite, the Fe polyhedra cross-link [Si2O7] groups into sheets. Keywords: fencooperite, unique crystal structure, hydrated barium–iron silicate–carbonate–chloride, ferric iron, five-fold coordi- nation, pinwheel. SOMMAIRE 3+ La structure cristalline de la fencooperite, dont la formule idéale serait Ba6Fe 3Si8O23(CO3)2Cl3•H2O, a été résolue en termes du groupe spatial trigonal P3m1, avec a 10.7409(5), c 7.0955(4) Å, V 708.9(1) Å3 et Z = 1. Le résidu final pour toutes les réflexions observées est égal à 0.038. La particularité de la structure de la fencooperite vient de la présence de deux modules structuraux non connus antérieurement. Des îlots de tétraèdres de silice forment des modules de stoechiométrie [Si8O22], que l’on peut décrire comme tétraèdres triples doublement et ouvertement branchés partageant un coin le long de [001]. L’autre module 3+ structural contient trois pyramides (Fe O5) ayant une des atomes d’oxygène en commun aux trois polyèdres, pour former un trimère en “moulinet à vent” de composition [Fe3O13]. L’absorption optique prononcée serait due à cette coordinence du fer. Ce trimère rattache les îlots [Si8O22] pour former la trame de la structure. Cette trame contient des canaux dans les trois directions cristallographiques, remplis par des atomes de Ba et de Cl et par des groupes de H2O et de CO3. La structure de la fencooperite montre des points communs avec celles de la gillespite, la pellyite, la titantaramellite et l’orthoericssonite. Dans la gillespite et la pellyite, des polyèdres individuels contenant le Fe2+ tétra-coordonné par l’oxygène rattachent les feuillets et les chaînes de tétraèdres de silice, tandis que dans la titantaramellite, les chaînes contenant le Fe3+ hexa-coordonnée par l’oxygène rattachent les anneaux d’îlots de tétraèdres de silice. Dans l’orthoericssonite, qui partage avec la fencooperite le Fe3+ en coordinence cinq, les polyèdres rattachent les groupes [Si2O7] en feuillets. (Traduit par la Rédaction) Mots-clés: fencooperite, structure cristalline unique, silicate–carbonate–chlorure hydraté de barium et de fer, fer ferrique, coordinence cinq, “moulinet à vent”. § E-mail address: [email protected] 1065 39#4-août-01-2256-09 1065 26/10/01, 14:57 1066 THE CANADIAN MINERALOGIST INTRODUCTION relevant to the data collection and structure determina- tion is given in Table 1. The three-dimensional data 3+ Fencooperite, ideally Ba6Fe 3Si8O23(CO3)2Cl3•H2O, were reduced for Lorentz, polarization, and background is a recently described mineral species (Roberts et al. effects and multiply-measured reflections were aver- 2001) from Trumbull Peak, Mariposa County, Califor- aged using the Bruker program SAINT. An empirical nia. The crystal-structure analysis was essential to the plate-absorption correction was done on the basis of description of this new species, as insufficient material 3998 reflections and reduced the merging R of this data was available to determine the valence of iron, and the set from 5.35% before the absorption correction to content of H2O and CO2. These three chemical param- 4.79% after it. eters were readily determined with a solution of the crys- All calculations were done with the Siemens tal structure. SHELXTL Version 5.03 system of programs, which Dunning & Cooper (1999) described the mineralogy incorporates scattering factors of neutral atoms taken of the barium silicates found at Trumbull Peak. In their from the International Tables for Crystallography, Vol. description, fencooperite is referred to as mineral C (1992). During the initial examination of the reduced UKTP–1, a Ba–Fe silicate. Further to this label, Dunning dataset to determine a starting space-group, sixteen pos- (pers. commun. 2001) states that: “Fencooperite occurs sible space-groups were considered (eight trigonal and exclusively in one lens associated with gillespite, san- eight hexagonal). Phasing of a set of normalized struc- bornite, titantaramellite, Fe–Cl analogues of ericssonite ture-factors gave a mean value |E2 – 1| of 0.800. This E and orthoericssonite, pyrrhotite, bigcreekite, alforsite, statistic is indicative of a non-centrosymmetric crystal barite, macdonaldite, a Ba–Fe–Mn–Ca silicate–phos- structure, which proved to be the correct interpretation. phate (Mineral 27 in Dunning & Cooper 1999) and with- A calculated sharpened Patterson function for space erite in a quartz matrix. In a second lens about 20 meters group P3 (the lowest symmetry possible among the six- to the south, the only minerals identified in this quartz- teen space-groups) located the two Ba sites and ten other rich lens are walstromite, pellyite, fresnoite, bigcreekite, lighter-element sites. This model refined to R = 0.174 benitoite, titantaramellite, a hydrous silicate (mineral 21 for observed data with Fo > 4␴(Fo). Additional O and C in Dunning & Cooper 1999), barite and some sanborn- sites were added following a series of ⌬F synthesis ite.” In the paragenetic sequence given by Roberts et al. maps, which, in turn, reduced the R index to 0.080 (ob- (2001), the crystallization of fencooperite follows bar- served data). In the final least-squares refinement, all ite, is contemporaneous with celsian, alforsite and likely atomic positions were refined with anisotropic-displace- titantaramellite, and precedes gillespite and sanbornite. ment factors to a residual of R = 0.048 (observed data in space group P3). The program MISSYM (Le Page X-RAY CRYSTALLOGRAPHY 1987) suggested the possible presence of a mirror plane, AND CRYSTAL-STRUCTURE DETERMINATION and space group P3m1 proved to be the correct choice of symmetry; the final refinement reduced R to 0.038 Precession single-crystal photographs initially (observed data). MISSYM (Le Page 1987) also indi- showed fencooperite to be hexagonal with diffraction cated that most of the structure is consistent with space symmetry 6/mmm (Roberts et al. 2001). It was not until group P6¯m2, thus it is not surprising that the diffraction the crystal-structure analysis was completed that it be- symmetry is consistent with 6/mmm. The addition of an came evident that fencooperite is trigonal (pseudo-hex- isotropic-extinction factor did not improve the results, agonal). Two sets of intensity data were collected on nor was there any evidence of merohedral twinning. The different crystals in an attempt to improve the refine- maximum and minimum electron-densities in the final ment. In fact, there was remarkably little difference be- cycle of refinement were +2.76 and –1.83 e–/Å3. Table tween the two refinements, but the marginally better 2 contains the final positional coordinates, anisotropic- experiment is described in full here. Comparing the two displacement factors, equivalent isotropic-displacement refinements, there are exact parallels, problematic aniso- parameters and bond-valence sums, and Table 3 con- tropic-displacement parameters, disparate bond-lengths in SiO4 tetrahedra and poor bond-valence sums. These problems are without doubt a signature of the poor crys- tallinity or granular nature of fencooperite. In the final data-collection, a fragment of fencooper- ite measuring 50 40 20 ␮m was mounted on a CCD-equipped Bruker P4 fully automated four-circle diffractometer operated at 50 kV and 40 mA. With the CCD detector, almost a full sphere of intensity data were collected out to 2␪ = 60° using a 120 s frame time and a crystal-to-detector distance of 40 mm. With these oper- ating conditions, no decrepitation was evident in the fi- nal analysis of the intensity standards. Information 1065 39#4-août-01-2256-09 1066 26/10/01, 14:57 THE CRYSTAL STRUCTURE OF FENCOOPERITE 1067 tains selected interatomic distances and angles. Ob- is 2.58 valence units, i.e., it is underbonded. The next- served and calculated structure-factors have been sub- nearest oxygen atom (O8) is at 3.22 Å, which does not mitted to the Depository of Unpublished Data, CISTI, increase the bond strength to any appreciable extent. The National Research Council of Canada, Ottawa, Ontario reason for the poor agreement between the calculated K1A 0S2, Canada. and the theoretical value of 3 is not clearly understood, DESCRIPTION AND DISCUSSION OF THE STRUCTURE There are nine cation sites in fencooperite: two cat- ion sites contain Ba, one Fe, four Si and two C.
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